This invention relates to transmissions equipped with load brakes. Specifically, this invention relates to transmissions utilizing the class of multiple-disk load brakes known generically as Weston brakes. Weston brakes provide automatic brake application in response to a load.
The Weston load brake has been used for many years. One of its unique characteristics is that it feeds back torque or rotational force to the multiple disk brake responsively to the torque or rotational force produced by the load to the brake. This feedback from the load to the brake permits the brake to automatically adjust to the force produced by the load. Optimally, the feedback system of a Weston brake provides smooth performance of the brake over a wide range of loading conditions and temperatures.
Historically, however, stable operation of Weston brakes has been difficult to attain. Under certain conditions of loading and temperature, Weston brakes are unstable and tend to chatter.
Often, this instability is the result of friction produced in the feedback device. One type of feedback device used with a Weston brake includes an input cam and an output cam--the input cam transmits motor torque to the output cam, and the output cam transmits the torque from the input cam to the load. A brake disk stack rotates with the load. The cams have opposing ramps where the cams engage one another. If the output cam tends to overrun the input cam, its ramps ride up over the corresponding ramps of the input cam. This action forces the cams to move axially apart, translating the output cam axially toward the brake disk stack. This axial motion of the output cam applies force between the output cam and the brake disk stack, compressing the brake disk stack, thereby applying a braking force. The greater the overrun, the greater the axial spread of the cams and the greater the braking force. Thus, greater braking force is created in response to the greater load.
Accordingly, the feedback system of the Weston brake depends on free axial movement of the output cam in order to apply force to the brake disk stack. In known applications of Weston brakes, the output cam is affixed to the output shaft by a conventional spline located on the output shaft, to provide continuous transmission of torque. The output cam moves axially along the spline to transmit various levels of axial force to the brake disk stack. A conventional spline transmits torque when the flanks of the teeth on one member press against the flanks of the corresponding teeth of the mating member. This pressure not only produces driving torque, it produces sliding friction as well, which impedes the ability of the output cam to adjust its axial pressure on the brake disk stack smoothly and continuously. Such uneven movement makes the brake unstable and causes chatter.
Movement of the cams transversely, or rotationally about an axis other than the shaft axis, also causes instability and chatter in the brake.
Viscous damping of the transmission by the introduction of transmission fluid is helpful in increasing stability of the brake and in limiting chatter. In some applications, however, transmission fluid may leak, which is undesirable.
Additionally, after repeated use of the Weston brake, the brake surfaces wear and the distance between the surfaces increases. When the brake surfaces have worn down sufficiently, the brake ceases to be effective and must be replaced.
In view of the foregoing, it would be desirable to provide a transmission equipped with a Weston brake that reduces instability of the brake over a wide range of loading and temperature.
It would further be desirable to provide a transmission equipped with a Weston brake in which the effective life of the Weston brake is extended.
It would still further be desirable to provide a transmission filled with a transmission fluid in which the leakage of the transmission fluid is substantially reduced.